Hydrostatic vehicle drive system comprising a differential lock

ABSTRACT

The invention relates to a hydrostatic vehicle drive system ( 1 ) comprising a first and a second hydraulic pump ( 2, 3 ), and a first and a second hydraulic motor ( 7, 12 ). Said hydraulic pumps ( 2, 3 ) and said hydraulic motors ( 7, 12 ) are coupled in such a way in a differential lock operation that a first connection ( 14 ) of the first hydraulic pump ( 2 ) is connected to a first connection ( 8 ) of the first hydraulic motor ( 7 ); a second connection ( 6 ) of the first hydraulic motor ( 7 ) is connected to a first connection ( 10 ) of the second hydraulic pump ( 3 ); a second connection ( 9 ) of the second hydraulic pump ( 3 ) is connected to a first connection ( 13 ) of the second hydraulic motor ( 12 ); and a second connection ( 11 ) of the second hydraulic motor ( 12 ) is connected to a second connection ( 5 ) of the first hydraulic pump ( 2 ). The absorption volume of the first hydraulic motor ( 7 ) and the second hydraulic motor ( 12 ) can be respectively adjusted by means of an adjusting device ( 20, 21 ).

The invention relates to a hydrostatic vehicle drive system comprising adifferential lock.

A hydrostatic vehicle drive system according to the preamble of claim 1is already known from DE 198 33 942 A1. Two hydraulic pumps arecross-connected to at least two hydraulic motors in the hydrostaticvehicle drive system which is disclosed in this publication, so that thefirst hydraulic pump, the first hydraulic motor, the second hydraulicpump and the second hydraulic motor are in each case arranged in seriesin differential lock mode in the working circuit. The series arrangementof the hydraulic pumps and hydraulic motors has the advantage of thesame volumetric flow flowing through both hydraulic motors and thelatter of necessity having the same output speed with the sameabsorption volume. This means that there is no slip at either of the twoseparate drive trains which are driven by the hydraulic motors. Thispublication also describes the changeover between the differential lockmode described above and an unlocked differential mode, in which thehydraulic pumps and the hydraulic motors are arranged in parallel withone another. However all the hydraulic motors which are described inthis publication are so-called fixed displacement motors with aprefixed, invariable absorption volume. Adjusting devices for adjustingthe absorption volume of the hydraulic motors are not used in thehydrostatic vehicle drive systems which are disclosed in thispublication.

The fact that the absorption volume of the hydraulic motors of thevehicle drive system described in DE 198 33 942 A1 is constant entailsthe disadvantage of the transmission ratio of the hydrostatictransmission, which is determined by the ratio of the absorption volumeof the hydrostatic motors to the volumetric delivery of the hydrostaticpumps, only being variable by changing the volumetric delivery of thehydrostatic pumps. The range of variation is therefore limited. Howevera large range of variation of the transmission ratio is desirable,especially for construction machinery, e.g. for front-end loaders, as arelatively low transmission ratio with a high torque is required whenworking on a building site, while a relatively high transmission ratiowith a high output speed is required for road transport.

The object of the invention is therefore to broaden the range ofvariation of the transmission ratio of a hydrostatic transmissionaccording to the preamble of claim 1.

The object is achieved by the characterising features of claim 1 inconjunction with the features constituting the preamble. The subclaimscomprise advantageous developments of the invention.

The proposal according to the invention lies in enabling the absorptionvolume of the—at least—two hydraulic motors in the working circuit to bevaried and providing the hydraulic motors with an adjusting device. Thisenables a greater range of variation of the transmission ratio of thehydrostatic transmission to be achieved.

A synchronizing mechanism which synchronizes the adjustment of theadjusting devices of the hydraulic motors is advantageously provided. Asthe hydraulic motors are arranged in series in differential lock mode inthe working circuit, the hydraulic motors should generally drive at thesame speed. This is only guaranteed if the two hydraulic motors have thesame absorption volume, this being ensured by the synchronizingmechanism. The synchronizing mechanism may comprise, for example, anelectrically operable servo valve, which is connected upstream of eachadjusting device, and an electronic control unit, which activates theservo valves of the hydraulic motors in synchronism. The electricalactivation simplifies the synchronizing process.

It is preferable to additionally provide an actuating pressureregulation system which increases the absorption volume of theassociated hydraulic motor when the pressure in the working circuitupstream of the associated hydraulic motor rises. The associatedhydraulic motor is thereby slewed further to produce a greater torquewhen the hydrostatic vehicle drive system is offered a resistance, forexample when a front-end loader moves the bucket into a heap of debris.The actuating pressure regulation system and the synchronizing mechanismcan in this case use the same servo valves.

The two hydraulic pumps are preferably only connected in parallel withone of the two hydraulic motors in unlocked differential mode. Thisincreases the range of variation of the transmission ratio. It ispossible to dispense with the additional torque of the second hydraulicmotor for road transport mode.

An embodiment of the invention is described in detail in the followingwith reference to the drawing. The sole figure shows a schematichydraulic circuit diagram of an embodiment of the hydrostatic vehicledrive system according to the invention.

The hydrostatic vehicle drive system according to the invention isdesignated as a whole by the reference character 1. A first hydraulicpump 2, which is formed as a variable displacement pump, a secondhydraulic pump 3, which is likewise formed as a variable displacementpump, and an auxiliary pump 4 are driven by a drive motor, for examplean internal combustion engine, which is not represented. The first andsecond hydraulic pumps 2 and 3 and the auxiliary pump 4 are preferablyarranged on a common drive shaft. Also provided are two hydraulic motors7 and 12, which are likewise formed so as to be adjustable.

A first connection 14 of the first hydraulic pump 2 is connected via aworking circuit section 18 to a first connection 8 of a first hydraulicmotor 7. A second connection 6 of the first hydraulic motor 7 isconnected via a working circuit section 17 to a first connection 10 ofthe second hydraulic pump 3. The second connection 9 of the secondhydraulic pump 3 is hydraulically connected via a working circuitsection 18 to a first connection 13 of a second hydraulic motor 12.However the second connection 11 of the second hydraulic motor 12 ishydraulically connected via a working circuit section 19 to the secondconnection 5 of the first hydraulic pump 2. The hydraulic pumps 2 and 3and the hydraulic motors 7 and 12 are therefore arranged in series withone another in differential lock mode such that a hydraulic pump 2, 3 isin each case followed in the hydraulic working circuit 16-19 by ahydraulic motor 7, 12 and, vice versa, a hydraulic motor 7, 12 is inturn followed by a hydraulic pump 2, 3. The flow delivered by thehydraulic pumps 2 and 3 is therefore of necessity routed via bothhydraulic motors 7 and 12, the later driving different drive trains, forexample different driving wheels.

The measure according to the invention effectively prevents slip ateither of the two drive trains, as the hydraulic motor 7, 12 connectedto the corresponding drive train is incorporated in the hydraulicworking circuit 16-19 such that the absorption volume of this hydraulicmotor 7, 12 cannot increase without increasing the volumetric deliveryof the two hydraulic pumps 2 and 3. Slip of the driven driving wheels oreven racing of the driving wheels is therefore counteracted.

Also to be seen in the drawing is a feed mechanism which is designatedas a whole by the reference character 22. The auxiliary pump 4 of thefeed mechanism 22 draws hydraulic fluid from a hydraulic fluid tank 23and feeds this into a feed line 24. The pressure in the feed line 24 isregulated to a constant level via a pressure limiting valve 25 connectedto the hydraulic fluid tank 23. The hydraulic fluid which is to bedelivered is fed into the sections 16-19 of the closed, hydraulicworking circuit 16-19 which carry the low pressure at the time.Provision must be made in this respect for the line sections of thehydraulic working circuit 16-19 which carry the high pressure and lowpressure to reverse when there is a reversal of the direction ofrotation of the hydraulic motors 7, 12 and therefore a reversal of thedirection of delivery of the two hydraulic pumps 2 and 3.

Pressure limiting valves 30 to 33 ensure that the pressure in the linesections 16-19 of the closed hydraulic working circuit which carry thehigh pressure at the time does not exceed a predetermined maximum value.If this maximum value is reached, the pressure limiting valve which isconnected to this line section will open and discharge hydraulic fluidvia the pressure limiting valve 25 to the hydraulic fluid tank 23.

Unlike a hydrostatic vehicle drive system of the type according to thepreamble, the hydraulic motors 7 and 12 are each provided with anadjusting device 20 and 21, respectively, by means of which theabsorption volume of the associated hydraulic motor 12 or 7 can beadjusted. For this purpose the section upstream and downstream of thehydraulic motor 12 and 7, respectively, is in each case connected via arespective connecting line 40, 41 and 42, 43, in which respectivenon-return valves 44, 45 and 46, 47 are arranged, to a direction ofrotation and changeover valve 48 and 49, respectively.

In the embodiment the direction of rotation-changeover valves 48 and 49can be electromagnetically activated and are connected to a direction ofrotation changeover unit 50 of an electronic control unit 51. When thedirection of rotation changes, the direction of rotation changeovervalves 48 and 49 are each changed over so that the section upstream ofthe hydraulic motor 12 and 7, respectively, carrying high pressure atthe time is connected to a connecting line 52 and 53, respectively. Thisconnecting line 52 and 53, respectively, is directly connected to afirst pressure chamber 54 and 55, respectively, of the adjusting device20 and 21, respectively, of the associated hydraulic motor 12 and 7,respectively. This connecting line 52 is connected on the other side viaa servo valve 56 and 57, respectively, and a non-return valve withparallel-connected throttles 60 and 61, respectively, to a secondpressure chamber 58 and 59, respectively, of the adjusting device 20 and21, respectively.

In the normal position which is represented the second pressure chamber58 and 59, respectively, is connected via the servo valve 56 and 57,respectively, to the hydraulic fluid tank 23. The servo valve 56 and 57,respectively, compares the working pressure prevailing upstream of therespective hydraulic motor 12 or 7, which is applied via the controlline 76 and 77, respectively, with a counterpressure which is exerted byan adjustable return spring 62 and 63, respectively. If there is a risein the pressure in the working circuit upstream of the respectivehydraulic motor 12 or 7, the second pressure chamber 58 or 59 will alsobe subjected to an increasing pressure, so that the associated hydraulicmotor is slewed further to a greater absorption volume and therefore agreater torque. This regulation is necessary, for example, if afront-end loader which is provided with a vehicle drive system 1according to the invention moves the bucket into a heap of debris andthe vehicle drive system 1 is thus offered increased resistance, so thata higher output torque is required. An operating mode of this kind isnot possible with hydraulic motors having a fixed absorption volume.

As the two hydraulic motors 7 and 12 are arranged in series in thehydraulic working circuit 16-19 in the differential lock mode describedabove, and the same volumetric delivery therefore flows through bothhydraulic motors 7 and 12, the slewing angle of the hydraulic motors 7and 12, which predetermines the absorption volume, must be synchronizedvia the adjusting devices 20 and 21, so that two hydraulic motors 12 and7 of the same construction are always subject to the same slewing angle.For this purpose the servo valves 56 and 57 additionally comprise arespective electromagnetic 64 and 65, by means of which the servo valves56 and 57 can be additionally acted upon against the return springs 62and 63. The electromagnets 64 and 65 are connected via electric controllines 66 and 67 to a synchronizing unit 68 of the electronic controlunit 51. Both electromagnets 64 and 65 are activated in the same way bymeans of the synchronizing unit 68 of the electronic control unit 51, sothat the same slewing angle and therefore the same absorption volume areobtained at the hydraulic motors 12 and 7 as long as the actuatingpressure regulation system described above, which monitors the pressureupstream of the hydraulic motors 12 and 7, does not respond. Theactuating pressure regulation system described above may also beprovided for both hydraulic motors 12 and 7 jointly. It is, however, ofadvantage to implement the actuating pressure regulation systemseparately for each hydraulic motor 12 and 7, for if just one drivetrain is blocked, only the pressure regulation system for this drivetrain responds.

The differential lock mode of the hydrostatic vehicle drive system 1 wasdescribed above. The embodiment which is represented in the drawing alsohas an unlocked differential mode. Differential lock changeover valves69 and 70 effect a changeover between differential lock mode andunlocked differential mode. The differential lock changeover valves 69and 70 are changed over by a differential lock unit 71 of the electroniccontrol unit 51. When the differential lock changeover valves 69, 70 arein the switched position 72 or 73, the hydrostatic vehicle drive system1 according to the invention is in the differential lock mode describedabove, in which the hydraulic pumps 2 and 3 and the hydraulic motors 12and 7 are arranged in series in the working circuit 16-19.

When the differential lock changeover valves 69, 70 are in theirswitched positions 74 and 75, the hydrostatic vehicle drive system 1according to the invention is in an unlocked differential mode. The twohydraulic pumps 2 and 3 are connected in parallel in this mode. Thefirst hydraulic motor 7 is additionally connected in parallel with thetwo hydraulic pumps 2 and 3, while the second hydraulic motor 12 isseparated from the working circuit 16-19. It would basically also beconceivable to connect the two hydraulic motors 7 and 12 in parallelwith the two hydraulic pumps 2 and 3. According to the development ofthe invention, however, it is proposed that just one of the twohydraulic motors 7, 12, this being the hydraulic motor 7 in theembodiment, be connected in parallel with the two hydraulic pumps 2 and3. This has the advantage of increasing the range of variation of thetransmission ratio of the hydrostatic transmission in the direction ofhigh speeds, as twice the volumetric delivery of the twoparallel-connected hydraulic pumps 2 and 3 is only opposed by the singleabsorption volume of just one of the two hydraulic motors 7. A highoutput speed and therefore a high transport speed is desired in roadtransport mode, while the output torque becomes insignificant, so thatthere is no need for the torque of the second hydraulic motor 12.

The invention is not restricted to the represented embodiment and can beused, for example, with a great many other configurations of theadjusting devices 20, 21. It is also possible to connect more than twohydraulic pumps 2, 3 and hydraulic motors 7, 12 in series indifferential lock mode or in parallel in unlocked differential mode.

1. Hydrostatic vehicle drive system (1) comprising at least one firstand one second hydraulic pump (2, 3) and at least one first and onesecond hydraulic motor (7, 12), wherein the hydraulic pumps (2, 3) andthe hydraulic motors (7, 12) are coupled together in differential lockmode such that a first connection (14) of the first hydraulic pump (2)is connected to a first connection (8) of the first hydraulic motor (7),a second connection (6) of the first hydraulic motor (7) is connected toa first connection (10) of the second hydraulic pump (3), a secondconnection (9) of the second hydraulic pump (3) is connected to a firstconnection (13) of the second hydraulic motor (12), and a secondconnection (11) of the second hydraulic motor (12) is connected to asecond connection (5) of the first hydraulic pump (2), characterised inthat the absorption volume of the first hydraulic motor (7) and of thesecond hydraulic motor (12) can be adjusted by means of a respectiveadjusting device (20, 21).
 2. Hydrostatic vehicle drive system accordingto claim 1, characterised in that a synchronizing mechanism (68, 64, 65,56, 57) which synchronizes the adjustment of the adjusting devices (20,21) of the hydraulic motors (7, 12) is provided.
 3. Hydrostatic vehicledrive system according to claim 2, characterised in that thesynchronizing mechanism (68, 64, 65, 56, 57) in each case comprises anelectrically operable servo valve (56, 57), which is connected upstreamof each adjusting device (20, 21), and an electronic control unit (68),which activates the servo valves (56, 57) of the hydraulic motors (7,12) in synchronism.
 4. Hydrostatic vehicle drive system according toclaim 2 or 3, characterised in that at least one actuating pressureregulation system (56, 48; 57, 49) is provided which increases theabsorption volume of the associated hydraulic motor (12; 7) when thereis a rise in the pressure in the working circuit (16-19) connecting thehydraulic pumps (2, 3) to the hydraulic motors (7, 12) upstream of theassociated hydraulic motor (20, 21).
 5. Hydrostatic vehicle drive systemaccording to claim 4, characterised in that the actuating pressureregulation system (56, 48; 57, 49) comprises a servo valve (56, 57)which is connected upstream of each adjusting device (20, 21) and thecontrol line (76, 77) of which is connection to the section (18, 19; 16,17), carrying the higher pressure, of the working circuit (16-19) whichconnects the associated hydraulic motor (7; 12) to the hydraulic pumps(2, 3).
 6. Hydrostatic vehicle drive system according to claim 5,characterised in that each servo valve (56, 57) is connected via anelectrically operable direction of rotation-changeover valve (48, 49) tothe sections (18, 19; 16, 17) of the working circuit (16-19) which areconnected to the associated hydraulic motor (7, 12).
 7. Hydrostaticvehicle drive system according to claims 3 and 5, characterised in thatthe servo valves (56, 57) of the synchronizing mechanism (68, 64, 65,56, 57) and the actuating pressure regulation system (56, 48; 57, 49)are identical.
 8. Hydrostatic vehicle drive system according to any oneof claims 1 to 7, characterised in that the first and the secondhydraulic pump (2, 3) is connected in parallel with one of the twohydraulic motors (7) and the other hydraulic motor (12) is disconnectedin differential unlocked mode.
 9. Hydrostatic vehicle drive systemaccording to any one of claims 1 to 8, characterised in that thehydraulic motors (7, 12) drive separate drive trains.